The present invention relates to a method for producing a transparent conductive film, whereby a transparent conductive film useful for e.g. a transparent electrode film for a liquid crystal display, a plasma display, an EL display, a touch panel or an anti-fogging glass for vehicles, or. an optical thin film useful for a building window having various optical properties, is formed by sputtering.
The present invention also relates to a method for forming a silicon oxide film having an alkali barrier property or a silicon nitride film having various functions.
As a method for forming the above transparent conductive film, there has been proposed a chemical film-forming method such as a sol-gel coating method, a spraying method or: a chemical vapor deposition (CVD) method, or a physical film-forming method such as a vacuum deposition method or a sputtering method.
However, for the industrial production of such transparent electrode films, the physical film-forming method such as a vacuum evaporation method or a sputtering method has been mainly employed for the reason that a transparent conductive film excellent in electrical properties and optical properties can thereby be readily obtained. Especially, in recent years, the sputtering method has been widely used for the-reason that a film can be formed uniformly on a substrate having a large area, and yet the film can be constantly formed even on a relatively low temperature substrate.
The sputtering method is a film-forming method, wherein argon gas is ionized by direct current (DC) discharge or radio frequency (RF) discharge and bombarded to a negatively biased metal or oxide target, and the substance sputtered from the target is precipitated on a substrate.
For example, when a tin oxide film is to be formed, tin oxide containing from 0.1 to 10 wt % of antimony or a tin alloy containing from 0.1 to 10 wt % of antimony may be used as the target material.
Likewise, when an ITO film is to be formed, indium oxide containing from 5 to 10 wt % of tin oxide (ITO target) or an indium-tin alloy containing from 5 to 10 wt % of tin (IT target) may, for example, be used as the target material.
Likewise, when a zinc oxide film is to be formed, zinc-oxide containing from 0.1 to 10 wt % of aluminum, zinc oxide containing from 0.1 to 15 wt % of gallium, a zinc alloy containing from 0.1 to 10 wt % of aluminum, or a zinc alloy containing from 0.1 to 15 wt % of gallium, may, for example, be used as the target material.
As the sputtering gas, an inert gas such as argon, which may contain oxygen, as the case requires, may be employed.
However, when the conventional sputtering method is used for the production of a transparent conductive film, in either case of using an oxide target or a metal target as the target material, there has been a problem such that during continuous sputtering, black fine protrusions (so-called nodules).which are believed to be a sub oxide having a remarkably poor sputtering rate as compared with the target material, are formed on the target surface, whereby the deposition rate of the transparent conductive film tends to gradually decrease, and at the same time arcing tends to occur frequently, whereby the target material spitted by the arcing is likely to deposit on the substrate and form defects of the transparent conductive film.
For example, if continuous sputtering is carried out for a long period of time by using a tin oxide target having a thickness of 6 mm, the deposition rate of tin oxide will decrease to a level of from 60 to 70% of the initial rate and at the same time, the frequency of arcing sharply increases, immediately prior to the completion of the use of the target.
Accordingly, in an industrial mass production process, it is common to gradually increase the sputtering electric power or gradually prolonging the deposition time, as the deposition rate decreases, to cope with the problem by experience. When the arcing frequency or the decrease of the deposition rate is especially remarkable, it is common to open the apparatus to the atmospheric air and to mechanically scrape off the nodules.
On the other hand, it has been known that formation of nodules can be suppressed by sputtering with a large electric power density. However, if the power density is increased, arcing occurs more frequently, and once such arcing occurs, the damage is likely to be more substantial.
With the above-mentioned target which is capable of forming a transparent conductive film, the target is likely to undergo cracking due to inadequate cooling of the target. Further, the sputtering speed tends to be high, whereby the film properties of the transparent conductive film tend to be poor.
To solve the above problems, it has been proposed to conduct cleaning for removal of the above nodules by plasma using nitrogen or a gas having a nitrogen component, as disclosed in Japanese Unexamined Patent Publication No. 293767/1992. This method has a merit that cleaning can be conducted without opening the apparatus to the atmospheric air, but it also has drawbacks such that, to conduct the above cleaning, the sputtering of transparent conductive oxide has to be once stopped, and if formation of nodules is remarkable, such nodules can not be completely removed even if cleaning is carried out for a long period of time.
It is an object of the present invention to provide a method for producing a transparent conductive film excellent in the productivity, which does not require cleaning by plasma or mechanical cleaning under the atmospheric air and whereby the above-mentioned conventional problems such as formation of nodules, generation of arcing and decrease of the deposition rate, can be solved.
In a liquid crystal display which has been rapidly progressed in recent years, it is known that alkali metal ions such as Na+ or K+ contained in the substrate glass diffuse through various thin layers formed on the surface of the glass substrate during the assembling process of the liquid display cell or during the use for a long period of time and reach the liquid crystal layer to deteriorate the performance of the device. Further, if such alkali metal ions reach the transparent conductive film such as ITO formed on the glass substrate, they deteriorate the electrical conductivity, which in turn deteriorates the response of the liquid display device or the quality of the Display. Therefore, when soda lime glass produced by a float method is to be used as the substrate for a liquid crystal display cell, it has been common to form a thin film of silica of a few tens nm as the first layer on the glass to let it have an alkali barrier property.
The ability to prevent the diffusion of alkali metals (i.e. the alkali barrier property) depends largely on the nature of the material to be used, and not only that, when a certain material is selected, the alkali barrier property depends largely on the density of microscopic defects or the impurity concentration in the material.
On the other hand, it is commonly known that if the substrate temperature is increased, the density of the thin film deposited on the substrate increases, and the density of the microscopic defects decreases. Accordingly, it is common to conduct film deposition on the heated substrate in order to obtain a thin film having a high alkali barrier property.
For these reasons, silica has been used frequently as a material for the alkali barrier film to prevent diffusion of alkali metals, and it has been common to conduct film deposition on the heated substrate, at a high temperature in order to obtain a high level of the alkali barrier property. Therefore, the substrate has been limited to a material having heat resistance.
The film-forming method may, for example, be a dipping method, a CVD method, an EB evaporation method or a sputtering method. From the viewpoint of costs and performance, the CVD method or the sputtering method is selected in many cases. As the CVD method, a so-called pyrolytic CVD method is employed, whereby the substrate temperature tends to be naturally high, whereby the restriction relating to the heat resistance of the substrate is rather severe.
On the other hand, in the sputtering method, it is basically unnecessary to raise the substrate temperature. However, in order to improve the alkali barrier property as mentioned above, or to reduce the specific resistance of the transparent conductive film during the subsequent sputtering process of the transparent conductive film, it is common to raise the substrate temperature to a level of at least 300xc2x0 C. From the viewpoint of the compatibility to such a process, it is usual to raise the substrate temperature to a level of about 300xc2x0 C.
Further, as a sputtering technique, it has been believed difficult to sputter Si stably by reactive sputtering. Therefore, it has been common to employ RF sputtering using a SiO2 target.
However, with RF sputtering, it has been believed difficult to apply large electric power or to form a film over a large area, and with a sputtering apparatus of a large scale, there has been a problem in the uniformity or in the deposition rate. Accordingly, it has been common to study the possibility of forming a SiO2 film at a high speed by DC sputtering which is believed relatively easy to apply for a large scale apparatus.
Accordingly, it is an object of the present invention to provide a method for forming a silicon oxide film having a sufficient alkali barrier property stably and yet at a low temperature by DC sputtering.
On the other hand, it has been found that silicon nitride may have an alkali barrier property equal or superior to that of silica. However, a so-called CVD method which comprises decomposing a molecular gas containing silicon and a molecular gas containing nitrogen by an exciting means such as heat or plasma and then reacting them to obtain silicon nitride, has a problem that the substrate temperature tends to be high during the film-forming operation. Further, it has a problem that during this film-forming operation, alkali metals from the substrate tend to diffuse into the silicon nitride film to deteriorate the alkali barrier property.
On the other hand, a so-called radio frequency sputtering method wherein silicon nitride or silicon is used as the target, and a silicon nitride film is formed by applying a radio frequency voltage to the target, has a problem that the deposition rate is low, and if input power is increased to improve the deposition rate, the temperature of the substrate increases, and electrons or negative ions bombard the substrate to cause damage and to let many defects form also in the growing silicon nitride film, whereby the alkali barrier property tends to be poor.
As described in the foregoing, with the conventional method for forming a silicon nitride film, it has been difficult to obtain silicon nitride having a sufficient alkali barrier property for the reason that the substrate temperature increases and so on, and it has been difficult to form a film stably at a high speed.
Accordingly, it is an object of the present invention to provide a method for forming a silicon nitride film having a sufficient alkali barrier property stably and yet at a low temperature by DC sputtering.
Further, several different methods are available for preventing permeation of moisture. Among them, however, it is usual to employ a method of laminating a thin film of an organic material having a low water permeability or a method of forming a: film of e.g. a metal as a protective layer on the material to be protected from moisture.
Among the above conventional methods for preventing permeation of moisture, the thin film of an organic material has a problem in the weather resistance because of deterioration due to ultraviolet rays. In the case where a metal is employed as the protective layer, the reflectance and the absorption coefficient of metal are generally large. Accordingly, such a metal can not be used in a case where a high level of optical transmittance is required.
The present inventors have found that the silicon nitride film has high water permeation resistance, excellent mechanical and chemical stability, and high transparency, which are very suitable properties for a protective layer. Further, by using a reactive DC sputtering method, a protective layer having a uniform thickness and uniform film quality can be formed at a high speed even for a substrate with a large area (Japanese Patent Application No. 184519/1993).
However, by a conventional reactive DC sputtering method, the resulting silicon nitride film is likely to change by a slight change in the deposition condition, and it has been difficult to form a silicon nitride film stably as designed.
Accordingly, it is an object of the present invention to provide a method for producing a silicon nitride film having a sufficient water permeation resistance stably by DC sputtering.
The silicon nitride film is useful also as an oxidation-resistant protective film (Japanese Unexamined Patent Publication No. 213632/1993).
As mentioned above, various methods are available for forming films of silicon nitride. However, the conventional CVD method has a drawback that the temperatures of the substrate and the material to be protected from oxidation will rise, whereby the substance to be protected from oxidation is likely to react. On the other hand, the conventional radio frequency sputtering method has a drawback that the substance to be protected is likely to be damaged by the bombardment of electrons or negative ions, and it is difficult to apply large electric power by a RF power source in order to accomplish the protection of a large area. Under the circumstances, the reactive DC sputtering method is being recognized as the best film-forming method.
However, as described above, with the conventional reactive DC sputtering method, the silicon nitride film is likely to be changed by a slight change of the condition, and it has been difficult to form a silicon nitride film stably as designed.
Accordingly, it is an object of the present inventions to provide a method for forming a silicon nitride film having a sufficient oxidation resistance stably by DC sputtering.
According to the first aspect, the present invention provides a method for producing a transparent conductive film composed mainly of an oxide by sputtering using a sputtering target capable of forming a transparent conductive film, wherein intermittent electric power is supplied to the target.
According to the second aspect, the present invention provides a method for forming a silicon oxide film having an alkali barrier property on a substrate by sputtering using an oxygen-containing gas as the sputtering gas and a sputtering target composed mainly of silicon, wherein intermittent negative voltage is applied to the target.
According to the third aspect, the present invention provides a method for forming a silicon nitride film on a substrate by sputtering using a nitrogen-containing gas as the sputtering gas and a sputtering target composed mainly of silicon, wherein intermittent negative voltage is applied to the target.